Distant metastases are the cause of approximately 90% of deaths due to breast cancer (BC). During BC development, novel properties are acquired by metastatic BC cells, allowing them to spread and escape immune system control. Recent studies on patients with early BC strongly suggest the involvement of NK cells in the control of tumor development. However, during metastasis, tumor cells may compromise the mechanisms of NK cell targeting and escape NK cell control. Therefore, towards identifying immune-related factors associated with BC aggressiveness, we analyzed microarray databases of human BC specimens and established BC cell lines. We specifically focused on interferon (IFN)-related transcripts due to the important impact of type I IFNs on anti-cancer responses. Importantly, we observed that the expression of IFN-lambda (IFN-?) receptor 1 (IFNLR1), the unique receptor for the newly discovered type III INF-?, negatively correlates with ER expression. Furthermore, we discovered a significant difference in the IFN-? response between primary and metastatic tumor cells. Specifically, in contrast to primary tumor cells, which lack IFNLR1 expression, metastatic cells express IFNLR1 and are responsive to IFN-?, as measured by STAT1 activation. Ex vivo culture of metastatic BC cells with IFN-? leads to accelerated in vivo metastasis. In addition, in mouse metastatic BC cells, we found that IFN-? down-regulates H60, a ligand for NK cell activation receptor NKG2D, and renders BC resistant to NK cell lysis. Our preliminary data indicate that the induction of IFNLR1 expression on BC cells is associated with the promotion of distal metastasis, which is fatal for BC patients due to a current lack of efficient therapy. Our data also indicate that an independent increase in endogenous IFN-? may fuel BC spread. We hypothesize that in addition to inhibiting NK cell tumor killing, IFN-? signaling in BC cells may induce tumor migration and invasion. IFN-?-induces NKG2D ligand down regulation and may afford metastatic BC cells with resistance to NK cell-mediated killing. To test the in vivo impact of IFNLR1 signaling on distal metastasis, we propose to generate murine and human BC cell lines with differential IFNLR1 expression and monitor their metastatic potential in immune competent mouse tumor models and immune-reconstituted human xenograft tumor models. We also propose to examine the impact of genetic ablation of IFNLR1 on in vivo distal tumor metastasis in syngeneic IFNLR1-/- MMTV-PyMT mice and in vivo silencing of IFNLR1 in our autologous immune-reconstituted patient-derived xenograft (AIR-PDX) model. To determine the molecular mechanism(s) underlying IFN-?-mediated downregulation of NKG2D ligands, we plan to investigate the effect of IFN-? on the inhibition of NKG2D ligand gene expression via miRNA or shedding of NKG2D ligands through the activation of matrix metalloproteinases. We believe that investigating the role of the IFN-?/IFNLR axis is crucial for developing new targeted therapies against metastatic BC.